1. Field of the Invention
The present invention relates to high temperature solid oxide electrolyte electrochemical cells and cell configurations and the electronic connection of such cells and configurations.
2. Description of the Prior Art
High temperature, solid oxide elecrtrolyte fuel cell configurations, and fuel cell generators, are well known in the art, and are taught by Isenberg, in U.S. Pat. Nos. 4,395,468 and 4,490,444. These fuel cell configurations comprise a plurality of individual, series and parallel electronically connected, axially elongated, generally tubular, annular cells. Each cell is electronically connected in series to an adjacent cell in a column, through narrow cell connections extending the full axial length of each cell. These connections contact the air electrode of one cell and the fuel electrode of an adjacent cell, through a conductive ceramic interconnection and an uncoated fiber metal felt current collector strip.
The porous felt strip, made for example of interlocking nickel fibers, bonded at contact points, extend axially between the cells. In the preferred embodiment air flows inside the cells and gaseous fuel outside. In the instance of reverse operation, with fuel flowing inside the cells, Isenberg in U.S. Pat. No. 4,490,444, and Isenberg et al. in U.S. Pats. Nos. 4,547,437 and 4,598,467, taught the use of conducting oxide fibers, such as doped In.sub.2 O.sub.3, as the felt material.
The nickel felt used in the preferred embodiment of the Isenberg patents is about 80% to 97% porous and is generally made according to the teachings of Brown et al., in U.S. Pat. No. 3,895,960, and Pollack, in U.S. Pats. Nos. 3,702,019 and 3,835,514, all involving the use of nickel fibers and matallurigical, diffusion bonding at fiber contact points, at about 900.degree. C. to 1200.degree. C., for use as battery plates.
In another type of fuel cell, involving liquid alkali electrolytes, as taught by Giner et al, in U.S. Pat. No. 3,513,029, a metal bi-porous electrode is used next to the electrolyte. This electrode contains three layers, where the layer contacting the electrolyte acts as an electrolyte barrier layer, and can be either conducting, made from noble metals, especially rhodium, conducting oxides and the like, or, in certain instances, insulating materials, such as cerium oxide, thorium oxide, silica, zirconium oxide, and magnesia oxide. In fuel cells more typical of the type used in this invention, thermally stable oxide fibers, such as yttria stabilized zirconia fibers, calcia stabilized zirconia fibers, alumina fibers, and alumina-silica fibers, have been used to strengthen the interior support structure for the fuel cell, as taught by Rossing et al., in U.S. Pat. No. 4,598,028.
In the preferred, high temperature, solid oxide elecrtrolyte fuel cells taught by the Isenberg patents, the low density, nickel-fiber felt strips which connect adjacent cells also provide the needed flexibility to conform to the cylindrical cell surface and to accommodate thermally-induced relative displacements of the cells, without losing electrical contact and without excessive mechanical loading of the cells. In the assembly operation, the nickel-fiber felt is compressed from its original interdiffusion bonded thickness of over 0.95 cm. (3/8 inch) to 0.16 cm. (1/16 inch). In this compression many of the nickel fibers are deformed or bent and thereby form new contacts with other fibers.
In operation at 1000.degree. C. the metal of these new fiber contacts will undergo diffusion and form new bonds in the fiber structure. These newly formed fiber bonds increase the rigidity of the fiber metal felt. Because of the increase in the number of fiber bonds, the fiber structure may lose its flexibility to the point where it will not be capable of maintaining an electrical connection between the cells during thermal transients. The increase in stiffness of the felt strips increases the forces applied to the cell electrodes.
Some problems associated with nickel felt strip adherence to the fuel cell interconnections was solved by electrochemical deposition of nickel metal on the non-porous, ceramic interconnection as taught by Isenberg in U.S. Pat. No. 4,648,945. This procedure, however, did not solve problems of stiffness and excess bonding in the nickel felt strip itself.